Soluble polyimides from aromatic dianhydrides and 10,10-di-(p-aminophenyl)thioxanthene

ABSTRACT

POLYIMIDES ARE PREPARED BY REACTING AN AROMATIC TETRACARBOXYLIC ACID DIANHYDRIDE WITH 9,9-DI-(P-AMINOPHENYL)THIOXANTHENE. THESE POLYIMIDES ARE SOLUBLE IN CERTAIN ORGANIC SOLVENTS WHEREBY THEY CAN BE FABRICATED INTO FILMS, COATINGS, LAMINATES AND THE LIKE. THE POLYIMIDES ARE ALSO FUSIBLE AND MOLDINGS THUS OBTAINED ARE LOW IN VOID CONTENT.

United States Patent 19 Claims ABSTRACT OF THE DISCLOSURE Polyimides areprepared by reacting an aromatic tetracarboxylic acid dianhydride with9,9-di-(p-aminophenyl)- thioxanthene. These polyimides are soluble incertain organic solvents whereby they can be fabricated into films,coatings, laminates and the like. The polyimides are also fusible andmoldings thus obtained are low in void content.

This invention relates to novel soluble aromatic polyimides which arecharacterized by a recurring unit having the following structuralformula:

i ma wherein X is and phenylene, where R and R are (lower)alkyl of from1 to 6 carbon atoms and aryl, and the polyamide acids from which theyare derived. The (lower)a1kyl group employed herein include bothstraight and branched chain alkyl groups having up to six carbon atoms.Examples of such groups are methyl, ethyl, propyl, isopropyl, butyl,pentyl, hexyl and the like.

The aryl groups of R and R include phenyl; phenyl substituted with oneor more alkyl groups, such as methyl, ethyl, propyl or with one or morehalogen groups such as chlorine or bromine; naphthyl, anthryl,phenanthryl and preferably phenyl.

The polyimides produced according to this invention are characterized byuseful solubility in certain organic solvents. Polyimides heretoforeknown to the art have generally been extremely insoluble, and have notbeen shapeable after conversion from the polyamide-acid state to thepolyimide form. The polyimides of the invention are extremely useful inthat they can be dissolved in certain solvents, in relatively highconcentration, and the solutions can be employed for further fabricationof the polyimides. In this way, it is possible to produce polyimidefilms, coatings and the like without the necessity of using apolyamide-acid intermediate with a follow-on converice sion step. Thisis highly advantageous, because it permits the application of polyimidecoatings to articles which might be damaged by heating or chemicalconversion techniques heretofore necessary.

The soluble polyimides display excellent physical, chemical andelectrical properties which render them capable of being used asadhesives, laminating resins for printed circuit boards, fibers,coatings for decorative and electrical purposes, films, wire enamels andmolding compounds.

These polyimides have been found to be soluble to the extent of at least20% by Weight at a temperature of about 25 C. in common polyamide typesolvents such as N-methylpyrrolidone, dimethylacetamide,hexamethylphosphoramide, dimethylsulfoxide, dimethylformamide and thelike, and solvents such as nitrobenzene and pyridine.

The polyimides are prepared by reacting an aromatic tetracarboxylic aciddianhydride of the formula wherein X is as defined previously, with9,9-di-(p-aminophenyl)thioxanthene having the formula H.N@C/@Nm @ii inan organic reaction medium which is a solvent for at least one of thereactants, preferably under substantially anhydrous conditions, at atemperature below C. and preferably at 20 C. to 50 C. The product ofthis reaction is a polyamide acid represented by the following formula:

no Z:@ @Ii OH i Mfg wherein X is as defined above.

The polyamide acid is subsequently converted to the polyimide by severalmethods which include heating the polyamide acid solution attemperatures between 100 C. and 240 C. depending on the boiling point ofthe organic solvent, until imidization is complete; by chemical meanse.g., by adding to the polyamide acid solution a dehydrating agent suchas acetic anhydride alone or in combination with a tertiary aminecatalyst such as pyridine and optionally heating or not heating theresulting solution at about C. until imidization is complete.

More specifically the preparation of the polyamide acid which issubsequently converted to the polyimide can be conveniently carried outin a number of ways. The diamines and dianhydrides can be premixed asdry solids in equivalent amounts and the resulting mixture can be added,in small portions and with agitation, to an organic solvent.Alternately, this order of addition can be reversed i.e. after premixingthe diamine and the dianhydride the solvent may be added to the mixturewith agitation. It is also possible to dissolve the diamine in thesolvent while agitating and to add slowly the dianhydride in portionsthat provide a controllable rate of reaction. However, this order ofaddition can also be varied. Still another process involves dissolvingthe diamine in one portion of a solvent and the dianhydride in anotherportion of the same or another solvent and then mixing the twosolutions.

The degree of polymerization of the polyamide acid and its correspondingpolyimide is subject to deliberate control. The mole ratio of diamine todianhydride in the initial reaction mixture may range from 2:1 to 1:2.The use of equirnolar amounts of the reactants under the prescribedconditions provides polymers of very high molecular weight while the useof either reactant in large excess limits the extent of thepolymerization. However, the scope of this invention includes both highand low molecular weight polyamide acids and their correspondingpolyimides.

The low molecular weight polymers can further be utilized asintermediate prepolymers which can be reacted with the appropriate chainextending agents to yield polymers which are useful as adhesives, and asmolding and laminating resins. The low molecular weight polymers may beend capped with reactive functional group compounds such as nadicanhydride, maleic anhydride, methylnadic anhydride and the like, andsubsequently heated to induce cross-linking and chain extension.

Besides using an excess of one reactant to limit the molecular weight ofthe polymers, a chain terminating agent such as phthalic anhydride oraniline may be used to cap the ends of the polymer chains.

To effect the conversion of the polyamide acids to the polyimides, thepolyamide acids are heated above 50 C. and preferably in an inertatmosphere and more preferably in an inert atmosphere between 110 C. and240 C. In the preferred process, the polyamide acids are prepared at atemperature below 50 C. and maintained at this temperature until maximumviscosity denoting maximum polymerization is obtained. The polyamideacid in solution and under an inert atmosphere is subsequently heated toabout 110 C. to 240 C. to convert the polyamide acid to the polyimide.The soluble polyimide may be alternatively prepared by mixing thediamine and the dianhydride at room temperature in a solvent such asnitrobenzene and then rapidly heating the mixture to reflux for about 2to 12 hours.

The starting aromatic diamines may be prepared by reactingthioxanthenone with aniline hydrochloride and aniline (1:4:10 mole ratiorespectively) at reflux temperature.

The soluble polyimides can be precipitated from their solutions by useof methanol, water, acetone, spray drying and the like. The resultinggranular material can be molded or redissolved in a suitable solvent toyield a filmforming or varnish type composition. Other appropriateingredients can be added to the polyimide solutions or molding powdersincluding fillers, dyes, pigments, thermal stabilizers and the like,depending on the end use.

It has also been found that these polyimides when heated above 225 C. inthe amtosphere will crosslink without the elimination of volatiles,yielding a polyimide which is essentially insoluble. Thus, thesepolyimides have the added advantage of being soluble and fusible duringthe fabrication stages and can be made insoluble if desired byappropriately heating the completely fabricated prodnot underatmospheric conditions at temperatures of greated above 225 C. in theatmosphere will crosslink withfrom oxidation reactions, although theexact nature of the cross-linking is not definitely known.

Thus, these polyimides are especially well suited for film, wire enameland laminating applications wherein the polyimides can be coated ontothe substrate from cold or hot solutions at a solids concentration offrom 25% to 50% by weight solids and cross-linked to yield coatingswhich are impervious to the solvents in which they were once soluble.

The quantity of organic solvent used in the preferred process need onlybe sufficient to dissolve enough of one reactant, preferably thediamine, to initiate the reaction of the diamine and the dianhydride.For forming the composition into shaped articles, it has been found thatthe most successful results are obtained when the solvent represents atleast 60% of the polymeric solution. This is, the solution shouldcontain ODS-40% of the polyimide component. The viscous solution of thepolymeric composition containing 10% to 40% polyimide in the polymericcomponent dissolved in the solvent may be used as such for formingshaped structures.

To further illustrate the nature of this invention and the processemployed in preparing the soluble polyimides, the following examples aregiven below:

EXAMPLE 1 9,9-dipaminophenyl thioxanthene A mixture of 5.2 g. (0.025mole) of thioxanthenone, 32.4 g. (0.25 mole) of aniline hydrochloride,and 9.3 g. (0.1 mole) of aniline was heated to reflux over one hour andmaintained at reflux for an additional hour. The cooled mixture waspoured into 10 ml. of ether and ml. of water. The mixture was filteredand the solids obtained was washed with ether, water, and ethanolrespectively. The product was recrystallized from methylCellosolve-water and had a melting point of 255 C. 261 C.

EXAMPLE 2 To a solution of 3.805 g. (0.01 mole) of 9,9-di-(paminophenyl)thioxanthene in 45 ml. of distilled N- methyl pyrrolidone, undernitrogen, was added 3.222 g. (0.01 mole) of 3,4,3,4'-tetracarboxylicbenzophenone dianhydride (BTDA) in portions over a 15 minute period. Thesolution was then stirred for about 15 hours at room temperature andunder nitrogen.

The reaction vessel was then immersed in a 200 C. oil bath. Thermalequilibrium was rapidly established at C., and the reaction mixture wasmaintained at that temperature for 3 hours. The reaction vessel wasswept out by a strong nitrogen flow during the first few minutes of theimidization so as to remove all traces of water formed in the reaction.The vessel was again swept out after 10 minutes, 30 minutes and 1 hour.

The soluble polyimide thus obtained had an intrinsic viscosity of 0.372in N-methylpyrrolidone at 30 C. and a glass transition temperature of 60C. as determined by torsional braid analysis.

Films were cast from the polyimide solution onto glass and aluminum andheated in a forced air oven at 200 C. for 1 hour to drive off thesolvent. The coatings obtained were clear, tough, and flexible and allthe coatings were able to be dissolved in the solvent from which theywere prepared.

When the same coatings were heated to 300 C. for one half hour, theywere still tough, clear and flexible; however, they were no longersoluble.

The polyimide was aged isothermally in a forced air oven at 300 C. Thepercent weight loss was minor after 600 hours.

The polyimide powder which was obtained by precipitation from solutionwith acetone and dried in a vacuum oven at 80, C. was soluble as a 20%solids solution in N-methylpyrrolidone, dimethylacetamide,dimethylformamide, hexamethylphosphoramide respectively.

EXAMPLE 3 To a solution of 3.805 g. (0.01 mole) of9,9-di(paminophenyl)thioxanthene in 45 ml. of distilled N-methylpyrrolidone, under nitrogen was added 3.222 g. (0.01 mole) of BTDA inportions over 15 minute periods. The solution was then stirred for about15 hours at room temperature and under nitrogen. To this solution wasadded ml. of acetic anhydride and 2.5 ml. of pyridine. The solution wasthen heated at 120 C. for 3 hours to yield the soluble polyimide.

The polyimide may be further converted to a molding powder bycoagulation from acetone with high speed stirring and drying the powderunder vacuum at 80 C. The powder may be molded by heating the powder ina cavity mold at 410 C. and a pressure of about 5000 p.s.1.

EXAMPLE 4 A solid mixture of 3.805 g. (0.01 mole) of9,9-di-(paminophenyDthioxanthene and 3.222 g. (0.01 mole) of BTDA in 100ml. of nitrobenzene was brought to reflux under nitrogen in one hour.The solution was maintained at reflux for 12 hours to yield a solublepolyimide. The polyimide solution may be applied directly as a coatingor a wire enamel useful, for example, as electrical wire insulation; or,the polyimide may be isolated by coagulation from acetone and used as amolding powder.

Laminates may be prepared from this soluble polyimide for use as printedcircuit boards by coating 7628 fiberglass cloth with the resin solution.Dry the coated fiberglass in a vacuum oven at 80 C. for 2 hours. Cut theprepreg into squares and lay up in a press preheated to 150 C. to form a13 ply laminate and hold for 15 minutes. Apply 500-1000 p.s.i. to thelaminate, then slowly raise the temperature to 215 C. Hold thistemperature and pressure for 2 hours, then increase over a 1 hour periodto 275 C. Hold at 275 C. for 1 hour, then heat to 360 C. and hold for 15minutes. Cool under pressure to 160 C.

EXAMPLE 5 By essentially following the procedure of Example 2, solublepolyimides may be obtained by reacting equivalent amounts of9,9-di-(p-aminophenyl)-thioXanthene and the following aromaticdianhydrides:

(a) bis(3,4-dicarboxyphenyl)ether dianhydride (b)bis(3,4-dicarboxyphenyl)sulfide dianhydride (c)bis(3,4-dicarboxyphenyl)sulfone dianhydride (d)bis(3,4-dicarboxyphenyl)methane dianhydride (e)1,l-bis(3,4-dicarboxyphenyl)ethane dianhydride (f)3,4,3,4-tetracarboxylic phenylbenzoate (g) 3,4,3',4'-tetracarboxylictriphenylamine dianhydride (h) 3,4,3,4-tetracarboxylic tetraphenylsilanedianhydride (i) 3,4,3',4'-tetracarboxylic tetraphenylsiloxanedianhydride (i) 3,4,3,4'tetracarboxylic triphenylphosphine oxidedianhydride (k) 3,4,3,4'-tetracarb0xylic triphenylphosphate dianhydrideWhat is claimed is: 1. A polyimide consisting essentially of therecurring unit and phenylene, wherein R and R are (lower) alkyl of from1 to 6 carbon atoms and aryl.

2. A polyimide according to claim 1, said polyimide consistingessentially of the recurring unit wherein R and R is lower alkyl of from1 to 6 carbon atoms or aryl.

3. A polyimide according to claim 1, said polyimide consistingessentially of the recurring unit ii \C 4. A polyamide acid consistingessentially of the recurring unit and phenylene wherein R and R are(lower)alkyl of from 1 to 6 carbon atoms and aryl.

5. A polyamide acid according to claim 4, said polyamide acid consistingessentially of the recurring unit i l i HOE @011 0/ wherein R and R arelower alkyl of from 1 to 6 carbon atoms or aryl.

6. A polyamide acid according to claim 4, said polyamide acid consistingessentially of the recurring unit 7. A solution of polyimide accordingto claim 1 in a 3 volatile solvent for said polymer.

8. A solution of a polyimide according to claim 2 in a volatile solventfor said polymer.

9. A solution of a polyimide according to claim 3 in a volatile solventfor said polymer.

10. A solution of a polyimide according to claim 9 wherein said solventis N-methyl pyrrolidone, dimethylacetamide, or dimethylformamide.

11. A solution of a polyamide acid according to claim 4 in a volatilesolvent for said polymer.

12. A solution of a polyamide acid according to claim 5 in a volatilesolvent for said polymer.

13. A solution of a polyamide acid according to claim 6 in a volatilesolvent for said polymer.

14. A solution of a polyirnide according to claim 13 wherein saidsolvent is N-methyl pyrrolidone, dimethylacetamide, ordimethylformarnide.

15. A self supporting film consisting essentially of at least onepolyimide according to claim 1.

16. A metal article coated with at least one polyimide according toclaim 1.

17. An article according to claim 16 in which the metal is copper.

18. A glass fabric or fiber impregnated with at least one polyimideaccording to claim 1.

19. A molding powder consisting essentially of at least one polyimideaccording to claim 1.

References Cited UNITED STATES PATENTS 3,539,537 10/1970 Holub 260783,534,003 10/1970 Holub 26078 3,661,849 9/1972 Culbertson 26047 CPMORRIS LIEBMAN, Primary Examiner R. ZAITLEN, Assistant Examiner US. Cl.X.R.

26032.6 NT, 65 R, 78 TF, 328

